This application claims the priority of Swiss application No. 948/96 and PCT/CH97/00147, filed Apr. 15, 1996 and Apr. 14, 1997, respectively, the disclosure of which are expressly incorporated by reference herein.
The present invention relates to a process treatment used for surfaces which are subsequently connected, to a cleaning process for such surfaces, to a connection process for such surfaces as well as to a workpiece pair and a workpiece stored in air during a defined time period.
Definitions:
Intimate "connection processes" are bonding (welding, soldering) as well as gluing. The surfaces may be metal surfaces, particularly Cu, Ni, Ag, Au, Pd, Si or plastic surfaces, for example, on an epoxy or ester basis. Such surfaces are often covered by oxides, nitrides or polyimides. The energy supply for the above-mentioned connections is basically of a thermal type, and the energy is supplied to the surfaces to be connected, for example, by means of heated tools, the Joule effect, UV radiation or preferably by ultrasound or by means of reaction energy when gluing.
"Passivation" or passivated: See Rompp's Chemical Dictionary, Franckh'sche Verlagshandlung Stuttgart, Edition 8, Page 3005. It is a bonded protective covering of the basic body surface. The clean basic body surface is protected from atmospheric air influences. This takes place, for example, by forming an oxide or nitride layer. For the establishment of a connection of the above-mentioned type, such a layer must first be opened up by energy which must be applied specifically for this purpose, for example, by applying higher temperatures than required for the actual bonding operation, or chemically, for example, by using a flux.
Conserving, which, in particular, requires no opening-up of the layer by additional energy during the bonding, differs fundamentally from the above-mentioned passivating. This conservation was recognized only in conjunction with and by way of the present invention which is used in a particularly advantageous manner when packaging integrated circuits.
During the so-called "packaging" of integrated circuits, several operations are distinguished which comprise a bonding of surfaces in the above-mentioned sense:
1. The individual integrated circuits are cut out of a silicon wafer and are applied to semiconductor system carriers (HLST) and are bonded with these (the so-called die bonding). The semiconductor system carrier surface is normally made of copper or of nickel, silver or gold or of a material on an epoxy basis, generally of a plastic material. Examples of such semiconductor system carriers are punched or etched metallic lead frames, ceramic substrates or ball grid array substrate carriers made of plastic. Hard soldering, soft soldering and gluing are used as bonding processes. In the case of flip chip solder processes, the integrated circuit is applied to a semiconductor system carrier by geometrically separated solder balls which are simultaneously used as an I/O-bonding.
2. Bonding the integrated circuits with contact support points on the semiconductor system carrier, as, for example, on the lead frame. The participating surfaces are metallic; for example, of Al, Au, Cu, Ni, Pd. Here, soldering or welding, particularly flux-free soldering or ultrasonic welding, are used as bonding techniques. This step is known as wire bonding.
3. Molding: In this process step, the circuits are molded by a molding material on the semiconductor system carriers, for example, the lead frames, after the wire bonding, the above-mentioned surfaces of the semiconductor system carriers and the integrated circuits participating with respect to the molding material.
Within the scope of a connection process for metallic surfaces, a cleaning process is known from EP-0 371 693, in which the surfaces, which subsequently are to be bonded in the above-mentioned sense only by the feeding of energy, are first exposed to a microwave plasma discharge in a hydrogen-containing vacuum atmosphere. Then, without an interruption of the vacuum, the provided solder layer is melted open by the plasma discharge for bonding the surfaces. Thus, by avoiding a contact with air, a contaminating surface coating, which would highly interfere with the subsequent bonding process, is avoided.
U.S. Pat. No. 5,409,543 describes the use of activated hydrogen for preparing a soldering operation. As a result, an oxide layer is opened up for the reaching-through of the soldering operation to the metal surface.
EP-A-0 427 020 describes a process which pickles, i.e, opens up a passive layer and oxide layer of joining partners by a preliminary high-frequency plasma treatment using a process gas. Gases or gas mixtures are used which, among others, consist of O.sub.2, H.sub.2, Cl.sub.2, N.sub.2 O, N.sub.2, CF.sub.4, etc.
If the above-mentioned pickling does not take place, immediately before the soldering operation, the joining partners are stored in protective intermediate storage devices, for which suitable receptacles, which are subjected to a protective gas, are provided for preventing contamination.
With regard to a first aspect, the present invention is based on the recognition that the cleaning process of the above-mentioned type, in an atmosphere, preferably in a vacuum atmosphere, with activated hydrogen, always leads to a conservation of the surfaces with respect to air. This conservation even permits the storing of the surfaces after the "cleaning" and before their bonding, in air, without the occurrence of the above-mentioned disadvantages with respect to the capacity to be bonded. Qualitatively, the bonding which takes place subsequently is not worse than if it had been established directly in the vacuum of the cleaning step or, generally without exposing the surfaces to air.
Often cleaning processes to be carried out for surfaces to be subsequently bonded in the above-mentioned sense are limited by the capacity of the participating parts to be thermally stressed. This applies particularly, if one group of surfaces are surfaces of the above-mentioned integrated circuits.
All processes of the above-mentioned IC packaging are critical, particularly if, for reasons of cost, certain semiconductor system carrier materials are to be used. Organic semiconductor system carrier materials should not be exposed to high temperatures. In addition, there is the tendency that the number of electrical connections of the integrated circuits increases constantly, while the cross-section of the connection wires is simultaneously reduced, for which a high reproducibility of the individual connections must be ensured.
With regard to a second aspect, in addition to a high connection reproducibility, an object of the present invention is to efficiently generate activated hydrogen but to simultaneously minimize the, thermal stressing of the treated surfaces. This is achieved in that, when a plasma discharge is used for activating the hydrogen, this is preferably implemented as a low-voltage discharge. The hydrogen can also, however, be activated in a different manner, for example, by the radiation of a defined energy or wavelength, particularly by UV radiation.
Definition:
A "low-voltage discharge" is a plasma discharge in the case of which a DC discharge is carried out by way of a cathode/anode path, the cathode being operated as a thermionic cathode (hot cathode).
With regard to a third aspect, the present invention relates to a connection process in which the surfaces, which were previously preferably cleaned in a vacuum process and were conserved according to the present invention, are exposed to air before they are subjected to the above-mentioned connection process. It is understood that this results in a high manufacturing flexibility. A workpiece pairing and a workpiece makes this possible.
Reference can be made to the following documents which are merely background:
Research Disclosure 307 107, wherein, in connection with a hardening process, a hard layer is produced by the action of an electron beam upon a surface;
GB-2 131 619, according to which mechanically loaded electric contact surfaces of switching contacts and a plasma discharge, preferably a high-frequency plasma discharge, are provided in an atmosphere containing hydrogen, oxygen, sulfur hexafluoride, etc.;
S. Veprek, et al., "Recent Progress in the Restoration of Archeological Metallic Artifacts by Means of Low-Pressure Plasma Treatment", from Plasma Chemistry and Plasma Processing, Vol. 8, No. 4, 1988, according to which surfaces of archeological metallic findings are conserved in a plasma discharge in a hydrogen-containing atmosphere;
K. Pickering, et al., "Hydrogen Plasmas for Flux-Free Flip-Chip Solder Bonding", from J.Vac.Sci.Technol. A8(3), May/June 1990, American Vacuum Society, according to which Pb--Sn soldering takes place in an atmosphere containing a microwave plasma in hydrogen;
V. Zhukov, et al., "Coadsorption of Hydrogen and Potassium on Silver Single Crystal Surfaces" from Pergamon, Vacuum, Volume 47, Number 1, 1995, Elsevier Science Ltd., according to which it was determined that precious metal surfaces are passivated by hydrogen adsorption;
N. Koopman, et al., "Fluxless Soldering in Air and Nitrogen", from I.N., 1993 Proceedings, 43rd Electronic Components and Technology Conference (Cat. No. 93CH3205/2), Proceedings of IEEE, 43rd Electronic Components and Technology Conference (ECTC'93) Orlando, Jun. 1-4, 1993, in which oxides on a solder surface are changed to oxyfluorides. As a result, a high passivation is achieved with respect to air so that, before the reflow bonding, surfaces treated in this manner can be stored in air for up to a week.